The present invention relates to a process for preparing a high-molecular-weight epoxy dimer acid ester resin from an epoxy resin and a dimer acid, an epoxy dimer acid ester resin prepared thereby, and a coating prepared from the epoxy dimer acid ester resin. More specifically, the present invention relates to a process for preparing a high-molecular-weight epoxy dimer acid ester resin in the homogeneous state, without an inclusion of a gel component, at a high production speed and high yield. The epoxy dimer acid ester resin is particularly useful for preparing a paint for forming a coating requiring a high water resistance, high corrosion resistance, high adhesiveness and high processability, especially a coating of a coil of a steel sheet which is subjected to draw forming or shear processing.
It is known that the flexibility or processability of an epoxy dimer acid ester resin is improved with an increase in the resins molecular weight. If during polymerization the proportion of a dimer acid to be reacted with an epoxy resin to an epoxy resin is high, as the polymerization is advanced, gelation often occurs during the polymerization. Even if gelation does not occur, the reaction mixture becomes viscous and it is difficult to obtain a highly polymerized product. Therefore, it has been very difficult to manufacture a high-molecular-weight epoxy dimer acid ester resin on a large scale.
Where, in order to prevent gelation, the polymerization is terminated before all the acid groups of the dimer acid react, the storage stability of the product is poor.
Conventional processes for the preparation of an epoxy dimer acid ester resin include (1) a process comprising reacting a three-component mixture of a liquid epoxy resin, a bisphenol and a dimer acid in the presence of a catalyst in an inert organic solvent such as a ketone type solvent such as methylisobutylketone or an aromatic solvent such as xylene, and (2) a process comprising dissolving a solid epoxy resin having a relatively high epoxy equivalent (for example, at least 1,000) in an inert organic solvent such as methylisobutylketone, and adding a dimer acid to the solution to effect the polymerization reaction.
In each of the above mentioned conventional processes, however, the concentration of the epoxy resin is relatively high and the solid content is at least 90 percent by weight, and for these reasons an abrupt increase of the resin viscosity occurs with an advance of the polymerization. If the amount of the dimer acid added to the resin is large, the entire mixture in a reaction vessel sometimes gels and it becomes impossible to increase the polymerization degree. Moreover, even if the solid concentration of the reaction mixture is reduced, the viscosity is considerably increased when the degree of polymerization is increased. Therefore, a paint prepared from the obtained resinous product will have unsatisfactory wettability with an article to be coated with the paint.
In the above-mentioned process (1), the presence of the bisphenol and dimer acid causes dehydration esterification and formed water deactivates the catalyst resulting in insufficient polymerization.
In the above-mentioned process (2), the dissolution of the solid epoxy resin must be conducted at a high temperature of 80.degree. to 100.degree. C. for several hours. Furthermore, because of the presence of a microgel in the solid resin, the paint prepared from this resinous product is unsatisfactory because of various characteristics, for example, extractability in an organic solvent, processability, and corrosion resistance.
It is desired to provide a process for preparing an epoxy dimer acid ester resin having a high molecular weight to an extent such that a processability comparable to that of a thermoplastic resin is obtained.